1. Field of the Invention
The present invention relates to electrical connectors and more particularly to high I/O density connectors such as connectors that are attachable to a circuit substrate or electrical component by use of a fusible element, such as a solder ball contact surface.
2. Brief Description of Prior Developments
The drive to reduce the size of electronic equipment, particularly personal portable devices and, to add additional functions to such equipment has resulted in an ongoing drive for miniaturization of all components. Miniaturization efforts have been especially prevalent in the design of electrical connectors. Efforts to miniaturize electrical connectors have included reductions in the pitch between terminals in single or double row linear connectors, so that a relatively high number of I/O or other signals can be interconnected within tightly circumscribed areas allotted for receiving connectors. The drive for miniaturization has also been accompanied by a shift in manufacturing preference to surface mount techniques (SMT) for mounting components on circuit substrates. The confluence of the increasing use of SMT and the requirement for fine pitch has resulted in designs approaching the high volume, low cost limits of SMT. The SMT limit is being reached because further reductions in pitch greatly increase the risk of electrical bridging between adjacent solder pads or terminals during reflow of the solder paste.
To satisfy the need for increased I/O density, electrical connectors have been proposed having a two dimensional array of terminals. Such designs can provide improved density. However, these connectors present certain difficulties with respect to attachment to the circuit substrate using SMT because the surface mount tails of most, if not all, of the terminals must be attached beneath the connector body. As a result, the use of two-dimensional array connectors requires mounting techniques that are highly reliable because of the difficulty in visually inspecting the solder connections and repairing them, if faulty.
Moreover, high terminal pin densities have made terminal pin soldering more difficult, particularly in SMT if there is a lack of coplanarity between the connector and the printed circuit board. In such a situation, some of the solder joints between the terminal pins and the PCB may not be satisfactory. As a result, reliability of the connector to circuit board connection may suffer.
Floating terminal pins have been proposed to allow the connector to adjust to any irregularities between the planarity of the connector and the circuit board. Some floating terminal pins have used a through hole in the connector body with a diameter about the size of the main terminal pin. However, because the through hole has to accommodate both the terminal pin and a stop that is typically pushed into the through hole during assembly, such designs can have dimensional tolerances that present manufacturing difficulties.
Other mounting techniques for electronic components have addressed the reliability of solder connections in hard to inspect positions. For example, integrated circuit (IC) mounting to plastic or ceramic substrates, such as a PCB, have increasingly employed solder balls and other similar packages to provide a reliable attachment. In the solder ball technique, spherical solder balls attached to the IC package are positioned on electrical contact pads formed on a circuit substrate to which a layer of solder paste has been applied, typically by use of a screen or mask. The assembly is then heated to a temperature at which the solder paste and at least a portion of the solder ball melt and fuse to the contact. This heating process is commonly referred to as solder reflow. The IC is thereby connected to the substrate without need of external leads on the IC.
While the use of solder balls in connecting electrical components, such as ICs, directly to a substrate has many advantages, some flexibility is lost. For example, for electrical components or ICs that are replaced or upgraded, removal and reattachment can be a burdensome process, since generally the solder connection must be reheated to remove the electrical component. The substrate surface must then be cleaned and prepared anew for the replacement electrical component. This is especially troublesome when the overall product containing the electrical component is no longer in the control of the manufacturer, i.e., the product must be returned, or a field employee must visit the product site in order to replace the component.
Of additional concern is thermally induced stress resulting from the effects of differential Coefficients of Thermal Expansion (CTE) between the electrical component and the circuit substrate. This susceptibility is primarily due to size, material composition and geometrical differences between an electrical component, such as an IC, and a circuit substrate.
Today's ICs, e.g., can perform millions of operations per second. Each operation by itself produces little heat, but in the aggregate an IC will heat and cool relative to the surface substrate. The stressful effect on the solder joints can be severe due to the differences in CTE between an electrical component and a circuit substrate. Even if the amount of heat generated at the interface portion between the substrate and electrical component remained relatively constant, differences in size, thickness and material of the substrate will generally cause the substrate and the electrical component to expand or contract at different rates. Further, nonlinearity in the rate of change of thermal expansion (or contraction) at different temperatures can further emphasize differences in CTE. These differences in expansion rates or contraction rates can place a burdensome stress on the solder joint, and consequently, an electrical component otherwise properly attached to a circuit substrate may still be susceptible to solder joint failure due to stress from varying CTEs.
This is of particular concern for ball type solder connections since the attachment surfaces are relatively small. Additionally, a circuit or wiring board can be very large relative to the size of a component. As a result, the effects from differences in CTE between components can be amplified. Further, since there is no additional mechanical structure, e.g. a pin, for added support, the stress on a solder joint is more likely to cause an electrical connection to fail, resulting in quality problems or rendering the electrical component inoperable. This phenomena is sometimes termed CTE mismatch, referring to the reliability and thus performance of electrical connections. The greater the differential displacements created by CTE mismatch, the greater is the concern for the electrical integrity of a system. Notwithstanding some loss in flexibility and difficulties due to differences in CTEs, the use of BGA and similar systems in connecting an IC to a substrate has many advantages.
In relation to BGA connectors, it is also important that the substrate-engaging surfaces of the solder balls be coplanar to form a substantially flat mounting interface, so that in the final application the balls will reflow and solder evenly to a planar printed circuit board substrate. Any significant differences in solder coplanarity to a given substrate can cause poor soldering performance when the connector is reflowed. To achieve high soldering reliability, users specify very tight coplanarity requirements, usually on the order of 0.004 inches. Coplanarity of the solder balls is influenced by the size of the solder ball and its positioning on the connector. The final size of the ball is dependent on the total volume of solder initially available in both the solder paste and the solder balls. In applying solder balls to a connector contact, this consideration presents particular challenges because variations in the volume of the connector contact received within the solder mass affect the potential variability of the size of the solder mass and therefore the coplanarity of the solder balls on the connector along the mounting interface.
BGA connectors have also been provided for connecting a first substrate or PCB to a second substrate or PCB, thereby electrically connecting the attached electrical components. For example,it has been proposed to secure half of a connector having a grid array of solder conductive portions to a first substrate by way of solder ball reflow, and by securing the other half of the connector having a grid array of solder conductive portions to a second substrate by way of solder ball reflow. This intermediate connector can absorb differences in CTE between the first and second substrate. Gains in manufacturing flexibility are also realized since the second substrate, with electrical component(s) attached thereto, can be removed and replaced easily. Since the second substrate is thus removable, it can be sized to match the electrical component. In this manner, CTE mismatch between the second substrate and the electrical component can be minimized.
However, even with the above described intermediate connector, it would be still further advantageous to provide a more flexible vehicle for electrically attaching an electrical component to a substrate that does not require replacing an entire second substrate, or that does not employ a second substrate at all, saving manufacturing time and materials.
Thus, there remains a need for an improved and more flexible apparatus and method for connecting an electrical component to a substrate that addresses the shortcomings of present electrical component connections, and also addresses the need to minimize or decrease CTE mismatch between an electrical component and a substrate.